The prismatic structure of the bivalve hinge ligament has been determined for Spisula solidissima. The prisms are of two types: both contain aragonite (50 to 60% dry weight) as the sole mineral form. The remaining material is protein, which is elastic when wet. One type of prism is cylindrical, having crystals oriented parallel to the prism axis, and in shape and orientation is similar to dental enamel prisms. The other type of prism is cylindrical but constricted or noded at regular intervals, and the crystal spacing varies regularly along the prism axis. Thus, the noded prisms probably respond to compression like an elliptical spring and account to the elastic properties of the ligament. Each aragonite crystal is surrounded by an organic sheath which is distinct from the bulk organic matrix. These sheaths have been isolated from the Spisula solidissima and Mercenaria mercenaria ligaments. The sheaths consist of protein only and have an amino acid composition different from their ligaments. The Mercenaria sheaths have an amino acid composition similar to the shell. The PIs of all major components of the sheath are less than 4.0 Work is in progress on the formation of CaCO3 in the presence of the sheaths and on the internal lamellar structure of the ligament prisms. BIBLIOGRAPHIC REFERENCES: G. Kahler, R. Sass, and F. Fisher. "The Fine Structure and Crystallography of the Hinge Ligament of Spisula solidissima." (1976) J. Comp. Physiol. 109. 209-220. G. Kahler, F. Fisher, and R. Sass. "The Chemical Composition and Mechanical Properties of the Hinge Ligament in Bivalve Molluscs." (1976) Biol. Bull., 151, 161-181.